<sect1 id="ai-zenith">
<sect1info>
<author><firstname>Jason</firstname> <surname>Harris</surname> </author>
</sect1info>
<title>The Zenith</title>
<indexterm><primary>Zenith</primary>
<seealso>Horizontal Coordinates</seealso>
</indexterm>
<para>The Zenith is the point in the sky where you are looking when you look <quote>straight up</quote> from the ground. More precisely, it is the point on the sky with an <firstterm>Altitude</firstterm> of +90 Degrees; it is the Pole of the <link linkend="horizontal">Horizontal Coordinate System</link>. Geometrically, it is the point on the <link linkend="ai-csphere">Celestial Sphere</link> intersected by a line drawn from the centre of the Earth through your location on the Earth's surface. </para><para>The Zenith is, by definition, a point along the <link linkend="ai-meridian">Local Meridian</link>. </para>
<tip>
<para>Exercise:</para>
<para>You can point to the Zenith by pressing <keycap>Z</keycap> or by selecting <guimenuitem>Zenith</guimenuitem> from the <guimenu>Location</guimenu> menu. </para>
</tip>
</sect1>
